The present disclosure sets forth a method and apparatus for installing a membrane in an electrolysis cell. A membrane is a somewhat delicate large sheet like, pliable member of relatively thin gauge spanning an electrolysis cell. A typical membrane is a plastic body having a thickness of just a few mils (less than about 5 mils) which is very substantial in size. In large cells, the membrane might be upwards of several meters in the two major dimensions. It is a thin sheet of plastic material which is rather pliant interposed between the two halves of an electrolysis cell. In the cell, opposing anode and cathode terminals are separated by the membrane. Perhaps a description of the well known chemistry occurring in an electrolysis cell will assist in describing the nature of membrane installation.
Consider perhaps the most popular electrolysis cell, namely, a large commerical size cell from the manufacture of chlorine gas and caustic. On one side of the cell, water with NaCl is introduced. An electric current flows across the membrane in the cell between the anode and cathode, the current initiating transfer of Na.sup.+ ions to the cathode side. That is, the membrane must be able to transport Na.sup.+ across the membrane. By direct inference, the membrane must be pervious to the migration of Na.sup.+ ions thereby suggesting that it have suitable ionic migration passages through it.
The membrane is placed between the two halves of the elctrolysis cell. In large size cells, they may be described generally as rectangular housing members which frame the membrane. So to speak, the membrane is stretched across one half of the cell and the other half is clamped against it. The two halves thus resemble rectangular frames with borders to define the internal cavity. The membrane must be clamped around the periphery so that the membrane fully spans, requiring all flow to be through the membrane, and thereby preventing leakage around the membrane. The membrane must maintain integrity, meaning there must be no tears or perforations in the membrane. Consider a typical large scale commercial electrolysis cell. Assume that the membrane when installed is as large as a bed sheet. Typically, the two rectangular frame members which clamp against the membrane must first be separated after draining the cell and the previous membrane is removed. After removal, the new membrane must be placed between the two halves. Assume that the two halves are quite large measuring several meters in the two major dimensions. The large frame members which comprise the electrolysis cell must be guided apart and guided back toward one another; this is because they are relatively heavy and must be mounted for movement on rails, guides, or tracks. They are separated only by a few centimeters. After separation, the new membrane is then placed between them and they are closed against one another. The membrane is positioned where it can be stretched taut in the fashion of a drum head. While high tension is not required, the membrane must be installed in a wrinkle free fashion. Based on the 1985 cost of membrane material, membranes can cost several thousand dollars. Even the tiniest snag will tear such an expensive membrane which really does not admit of repair. Moreover, the requirement for safe installation dictates the necessity that the membrane be installed free of wrinkles and pulled taut with a measurable degree of tension. Excessive tension cannot be permitted; hand held tension is perhaps difficult to control.
The membrane is normally supplied in sheet stock. So to speak, it can be supplied in a spool of material and cut to size. Even so, this does not prepare a membrane for instantaneous installation. Rather, the membrane is initially moistened in most instances. The manner and mode in which this is accomplished may vary depending on the material of the membrane and the nature of the electrochemical process undertaken in the cell. Using as one example a membrane manufactured by DuPont and sold under the trademark Nafion, this sheet like member typically measuring less than about 5 mils in thickness must first be soaked. It must be soaked in caustic preliminary to installation in a chlor-alkali cell. Soaking weakens the membrane structurally. That is, it is more resistant to puncture or tear when it is dry. Yet, it must be installed after wetting. This poses an additional problem in handling. One approach heretofor used simply required substantial hand labor as many personnel grab the membrane around the edges and guide it into a horizontal posture to enable the membrane to be transported from the soaking facility to a location where it is then hung vertically in the cell. In most instances, the membrane must be installed hanging vertically between the two cell halves which frame or bracket the membrane. By hand transport, the membrane, soaked in caustic, must be gently deployed with one edge dropping in the narrow slot between the two halves of the electrolysis cell. This is no easy feat, namely gently deploying a delicate sheet membrane of substantial size (almost negligible weight) where the bottom edge is lowered gently through a relatively narrow slot between facing rectantular frame members. Once it has been positioned vertically, it still must be properly aligned to assure that the marginal surplus around the four edges is properly distributed. That is, the membrane must be located so that a slight marginal excess is located around all four sides of the rectangular electrolysis cell. This may require last second positioning adjustment. The repositioning necessary to accomplish this runs the risk of distorting the shape of the membrane with wrinkles or excessive tension. It is hard to do with hand held membrane positioning procedures.
The method of the present disclosure enables the membrane to be handled from the time of wetting to the point at which it is stretched vertically in the narrow confines of an electrolysis cell, suspended wherein the lower edge hangs below the bottom of the cell and yet where the top edge extends above the top of the cell. This is an installation procedure which enables the delicate membrane to be transported, installed and stretched without the risk of hand held procedures. It is a procedure showing great advantage over devices known in the prior art which do not suggest the method of installation herein disclosed. For instance, U.S. Pat. No. 2,311,245 sets forth a clamping assembly used in stretching or drying curtains or other fabrics. However, given the sense of clamping taught by this structure, there is no suggestion whatsoever of the procedure as will be described. Another exemplary reference is found in U.S. Pat. No. 4,147,257. This reference also shows a marginal edge clamping structure. Of similar import, U.S. Pat. No. 3,364,528 also shows a multiple sheet clamping device.
By contrast, the method and apparatus of this disclosure set forth a method and means for handling delicate pliable sheet membranes of large dimension with a view of spooling a wet membrane onto a transport cylinder where it is wrapped around the transport cylinder in multiple wraps for ease of transportation, thereby enabling the transport cylinder to be moved from the location where the membrane is stored when wet and the location of the slightly opened electrolysis cell. In addition to the transport cylinder, two parallel edges of the membranes are clamped by stiff marginal clamps extending longer than the dimension so clamped, thereby extending at the ends. The clamps are equipped with end located handles for ease of manipulation. The two edges are then deployed with respect to the transport cylinder, one being stored within the cylinder so that the membrane is rolled around the cylinder for ease of transportation. Over the electrolysis cell, it is unrolled with the lower edge dropped through the electrolysis cell, thereby positioning the membrane in the electrolysis cell with a controlled measure of tension determined by the weight of the clamp member across the bottom edge. This enables the device to be used to momentarily transport and then deploy the membrane whereupon the membrane is clamped by closing the two halves with the cell and then the transport cylinder and clamps can be removed from the membrane after installation on closing the cell halves.